Dogs with cyclic hematopoiesis (CH) experience 12-day repetitive cycles of bone marrow cell production. We propose to evaluate progenitor cells committed to erythroid and granulocytic development with in vitro studies of colony formation of these committed cells. This should determine if all cell progenitors are cyclic at the same time. Marrow will be separated into cellular components and evaluated as to colony forming ability in the two systems. Cell fractions and various preparations will be incubated to determine if cell-cell interactions within the marrow are inhibiting or enhancing colony formation in vitro.